This invention relates to electric motor controls, and more particularly to devices and methods for determining the position of a rotor of an electric motor.
In drive-based control of permanent magnet synchronous machines (PMSM), knowledge of rotor position is required to control electromagnetic torque. In practice, numerous methods of measuring or estimating position are applied. Traditionally, for machines that have a trapezoidal back-emf, 120° square wave excitation is utilized. Under 120° excitation, three Hall-effect sensors placed at 120° intervals provide sufficient knowledge of position to achieve control of speed or average torque. For machines that have a sinusoidal back-emf, applied excitation it typically sinusoidal. Therefore, more precise knowledge of rotor position is required. Over the past several years, significant effort has been placed on the use of discrete Hall-effect sensors to estimate rotor position for sinusoidal PMSMs [1-8]. Research has focused primarily upon the integration of an estimated rotor velocity to interpolate the rotor position between sensor transitions. Specifically, in [1-5], a zero-order Taylor series approximation of the integral is used. In other words, the average rotor speed is calculated from the previous sensor state and is integrated to determine rotor position between sensor transitions. In [6-8], a first-order Taylor series approximation of the integral that uses the average rotor speed from the previous sensor state and the average angular acceleration calculated from the previous two sensor states is used to determine the rotor position. In [1-4, 7-8], 120° square wave excitation is commanded versus sinusoidal excitation for start-up and low-speed operation.
In [5-6], observers are presented that utilize a single Hall-effect sensor for position estimation. Although effective at higher speeds, the error bound at zero speed is on the order of ±90° electrical. Therefore, in [5], the motor is started using a sinusoidal excitation with a fixed frequency until the rotor has achieved sufficient speed. In [6], the motor is aligned to a known position and then started on a pre-determined position profile.
Although, the methods in [1-8] have shown reasonable success, they have limitations. Specifically in [1-4, 7-8], the use of three Hall-effect sensors limits the guaranteed starting torque to 87% of maximum starting torque. In [5-6], the use of a single Hall-effect sensor further reduces the guaranteed maximum starting torque. Specifically, in [6], the synchronous excitation used during start-up can result in low starting torque and in an initial reverse rotation of the rotor. For the start-up strategy used in [6], a guaranteed starting torque cannot be determined without knowledge of the rotational inertia of machine and load and the synchronous excitation frequency. In [7], the force alignment of the rotor during start-up can result in reverse rotation of the motor, and the guaranteed starting torque is less than 71% of the maximum starting torque available.
Although there is some reduction in performance, Hall-effect-based position observers provide a drive system designer with a tool for reducing cost (i.e. compared to using more expensive in-line techniques). Moreover, reduced-count observers provide an additional advantage in that they improve a drive's fault tolerance. Thus both have found widespread use in numerous applications including automotive actuators, home appliances, and industrial equipment. Seemingly unrelated to Hall-effect-based position observers, a topic that has also received considerable attention recently is the mitigation of torque ripple in PMSMs. Specifically, in many of the applications that call for a reduction in drive system cost, an additional constraint is added that the acoustic noise/vibration created by the drive must be eliminated (or at least minimized). In PMSM drives, a dominant source of noise and vibration is created by torque ripple that results from stator/rotor field interaction as well as cogging torque.
Over the past decade several controllers have been proposed that adjust stator current harmonics to mitigate torque ripple [9-33]. However, the majority of the techniques require knowledge of machine parameters (back-emf and cogging torque coefficients). Moreover, validation of torque ripple mitigation controllers has typically used in-line high performance (relatively costly) position sensors. This has led to a common perception that torque ripple mitigation requires the use of a precise position encoder.
Recently, a series of studies have shown a possible path toward cost-effective control-based torque-ripple mitigation in mass-produced drives [34-36]. Specifically, to eliminate the need to obtain knowledge of machine parameters, a piezoelectric polyvinylidene fluoride (PVDF) polymer film is used in [34] to detect torque-ripple-induced vibration. The advantage of the sensor is that it is relatively low cost and straightforward to implement in a drive system. Using the sensor for torque ripple feedback, a mitigation algorithm based upon cost-function minimization is developed in [35]. The controller is applicable for machines with arbitrary back-emf and togging torque waveforms. In [36], vibration created by torque ripple has been shown to be useful in predicting initial rotor position.